The invention concerns a safety system for an electrohydraulically controlled automatic transmission preferably driven by an internal combustion engine and whose reduction ratio is determined via a shift characteristic field by an electronic control unit depending at least on a throttle valve position and a transmission output speed wherein a driver can engage the control of the automatic transmission by means of a selector lever.
It is known from the prior art that a driver prevents certain shifts by the position of a selector lever. For example, if the selector lever is in the three position, then shifts to the fourth and fifth gears are prevented. Typical cases of this are downhill drives. As relevant prior art can be taken into consideration in this connection, e.g., the book Pkw-Automatgetriebe by H. Dach and P. Kopf, Moderne Industrie Editing House, 1994, pages 58-61.
In the practice the following problem can now arise: If a driver at the end of the downhill drive inadvertently moves the selector lever from the three position to the neutral position, then there exists no more frictional connection between the transmission output and the transmission input. If at the same time the driver has actuated the accelerator pedal, then the speed of the internal combustion engine will very quickly increase in relation to the governed speed. If the driver now detects his error, he will try to shift the selector lever to a forward drive position. In other words, in the first shift from the three position to neutral, the clutch for the third gear will be opened. In the second shift from the neutral position to a forward drive position, the clutch for the third gear again closes. In this closing process, however, the speed of the internal combustion engine or the transmission input speed is forced from a high level to a low level, hence the third gear here. This elevated speed difference leads to a strong loading of the engaging clutch.
In view of the above, the invention is base on the problem of reducing the load of the engaging clutch for the above described case.
According to the invention, the problem is solved by the electronic control unit testing while driving whether there has occurred a first change on the selector lever from a drive position to a neutral position followed by a second change from the neutral position to the drive position, and the electronic control unit determining a virtual reduction ratio upon detection of the first change and the selector lever being in neutral position. The virtual reduction ratio is calculated from the quotient of the transmission input speed to the transmission output speed. With detection of the second change and a selector lever being in a drive position, the electronic control unit compares the virtual reduction ratio with the reduction ratio resulting from the shift characteristic field. In case of a divergence, the virtual reduction ratio is assigned to a characteristic field which has lines of constant reduction ratio surrounded by a reduction ratio range. As a consequence of this, the electronic control unit will determine a set reduction ratio whose range contains the virtual reduction ratio. The solution, according to the invention, offers the advantage that the same clutch, such as for the third gear, is not first opened and then closed again. The case can arise, that instead of the clutch for the third gear, the clutch for the second gear is closed. Thereby becomes reduced the speed difference or the shifting work of the engaging clutch.
In development of this, it is proposed that the reduction ratio ranges do not overlap and the electronic control unit forms a difference from the virtual reduction ratio and the possible reduction ratios of the automatic transmission when the virtual reduction ratio is between two ranges. As a consequence of this, the electronic control unit will select a set reduction ratio in which the difference results in a minimum.
It is proposed that upon detection of the first change and the selector lever being in the neutral position, the further curve of the virtual reduction ratio is forecast. In development of this, it is proposed that, at a first and second moment, a transmission input speed value be found and a gradient of the transmission input speed be determined therefrom. From the gradient of the transmission input speed, at the first moment, is now calculated a transmission input speed value that is to be expected at a third moment. From the transmission input speed value expected, the virtual reduction ratio is then forecast at the third moment. This development, according to the invention, takes into consideration the case when the driver immediately after having activated the neutral position, detects his error and simultaneously releases the accelerator pedal and shifts the selector lever to a forward drive position. Due to the release of the accelerator pedal, the speed of the internal combustion engine, or the transmission input speed, quickly becomes reduced. Since signal transit times appear between the issuance of a shift command by the electronic control unit and the actual closing of the clutch, it is possible, by applying the solution according to the invention, to obtain the advantage of a more accurate selection of the set reduction ratio.